Can you remember if you have moved the Plex data folder away from its default location?
If you did, verify that the drive where it was moved to still has the same drive letter as it did under Windows 10.
(If you cannot remember what drive letter it was, take a look into the registry and see where the relevant key is pointing to [HowTo] An extended guide on how to move the Plex data folder on Windows )
I hope you didn’t switch from a local user account to a Microsoft account during the OS upgrade?
